The present invention relates to a high availability or fault tolerant memory system for a computer system. Such a computer system may be employed in telecommunications network switches, where high availability and fault tolerance is required.
In such memory systems, memory is often duplicated in shadow or mirror shared memory cards so that system performance is not halted or degraded by an error or fault in one shared memory card. It may be necessary in these systems to discover when a shared memory card has an error or fault and to diagnose the error or fault. As well, the systems need the capability to add or remove redundant shared memory cards, in order to enhance fault tolerance or eliminate excessive redundancy.
A common difficulty with systems having redundant shared memory is keeping track of how many redundant copies of data are stored and allowing a variable number of redundant copies to be stored. Many systems require a fixed number of responses to all memory requests. This reduces system flexibility and fault tolerance.
U.S. Pat. No. 4,995,040 issued Feb. 19, 1991 to Best et al. discloses a fault tolerant, fail-safe computer system including a management unit and buffer to manage asynchronous redundant digital messages, to sort the redundant messages and to compare redundant messages. The messages are compared by a majority voting scheme. The system disclosed by Best may provide high integrity but is unlikely to provide high availability or high reliability.